The engine has a carburetor for injecting fuel and mixing the same with air. A carburetor conventionally has a means which is controlled mechanically to provide a desired air/fuel ratio. But a carburetor currently used is attached to additional devices for purifying exhaust gas and, for this reason, is complicated. The mechanically complicated carburetor impairs the reliability of the air/fuel ratio control and does not readily perform the matching necessary for maintaining a highly accurate control. Accordingly, the conventional mechanical control type of carburetor requires an improvement in order to maintain a highly accurate air/fuel ratio control with those engine control systems currently being used.
Additionally, in current engine control systems, use is being made of a microcomputer and the microcomputer may also be applied advantageously for air/fuel ratio control in the carburetor.